sect 23 transfer mold design tips

7/27/2019 Sect 23 Transfer Mold Design Tips

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P L A S T I C S E N G I N E E R I N G C O M P A N Y S H E B O Y G A N , W I S C O N S I N 5 3 0 8 2 - 0 7 5 8 U . S . A

3518 LAKESHORE ROAD

POST OFFICE BOX 758

PHONE 920 - 458 - 2121F A X 920 - 458 - 1923

Thermoset Transfer Mold Design Tips

When designing a mold for a transfer molded part, it is important to keep in mind that the goal is

produce parts with the best quality, in as short a cycle as possible, with a minimum of scrap. To

achieve this goal, you will need a mold that has a uniform mold temperature, has balanced fill,

and is properly vented.

MOLD HEATING

A uniform mold temperature means that the temperature of each half of the mold is the same(within 3C (5F)) for all locations when the mold is heated by oil or steam. Molds that are

heated with electric cartridge heaters can vary by as much as 6C (10F). A mold with a uniform

temperature will fill easier and produce parts with less warpage, improved dimensional stability

and a uniform surface appearance. Achieving a uniform mold temperature is dependent on your

method of mold heating.

A mold that is heated by steam or oil will have a uniform mold temperature because the heat

source maintains a constant temperature. However, oil, as a heat source, is only about half as

efficient as steam. Therefore, when using oil to heat a mold, it is necessary to set the oil

temperature higher than the desired mold temperature.

Electrically heated molds are more difficult to maintain at a uniform temperature because the

cartridge heaters are constantly cycling on and off. When they are on, they generate a great deal

of heat at the source but this heat must be distributed throughout the mold in a way that produces

a uniform mold temperature.

To determine the amount of wattage needed to heat a mold, the use of the following formula

might be helpful: 1 kilowatts for every 45kg (100 pounds) of mold steel. Note: This

formula normally will allow the mold to be heated to molding temperatures in 1 to 2 hours.

Locating a heater on the centerline of the mold is not recommended, because the center of themold is normally hot enough without adding any additional heat. Typically, the cartridge

heaters are located in the support plates, with a distance of 65 mm (2") between heaters.

NOTE: Deep draw molds may need to also have heaters in the retainer plate. There should be a

minimum of one thermocouple to control each half of the mold. In larger molds, it is

recommended to have more than one thermocouple in each mold half. This will result in better

control and more uniform mold temperatures. The thermocouples should be located in the A

and B plates, between two heaters if possible and at a distance of 32 mm - 38 mm (1" - 1")


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from the closest cartridge heater. This distance is to be measured from the edge of thermocouple

hole to the edge of the cartridge heater hole. The distance from the thermocouple to the heater is

important because a heater that is too close will cause the thermocouple to turn off the heat

before the mold is at temperature. A heater that is too far away from the thermocouple will result

in a mold that overheats and then gets too cool. Likewise, it is not a good practice to position a

thermocouple so it senses the external surface temperature of the mold. If possible, it should be

located 38 mm - 51 mm (1" - 2") inside the mold, since the temperature taken there, is lesssusceptible to outside influences and therefore more stable.

BALANCE MOLD FILL

When transfer molding multiple cavity molds, it is important that all the cavities are filled

simultaneously. The most common method to achieve a balanced fill is to make the runner

length from the transfer pot to each cavity equal. This approach will work as long as the material

flows directly from the transfer pot to the gate of the part. However, if the runner is divided two


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or three times between the pot and the gate, it is unlikely that the fill will be balanced. An

effective way ofbalancing the fill is to have one main runner that extends from the last cavity

on one end of the mold to the last cavity on the opposite end, with sub-runners feeding the

individual cavities. To balance the fill of the cavities, flow restricter pins are placed in the

subrunners. These pins are adjusted to inhibit the flow of material to the individual cavities so

all the cavities are filled at the same time.

VENTING

When molding thermosets, the polymerization process that takes place produces volatiles, which

along with the air already within the cavity chamber can become trapped and superheat to 375C -

425C (700 - 800F). If the gases are not allowed to escape through vents, they may oxidize the

lubricants leaving burn marks on the part. The vents allow the volatiles to escape to atmosphere.

In addition to visual problems, improper venting will result in parts that cannot be filled, have

dimensional problems, or have less than the expected physical and/or electrical strengths.

The first question that has to be addressed is vent location. It is important that all vents must leadto the atmosphere, otherwise the vent will be useless. Unless the part geometry shows some

obvious locations for vents, a brief molding trial should be conducted to observe where the gas

voids occur. Whenever possible, vents should be located in the moveable half of the mold,

wherever a gas void or knitline is seen on a part.

Vents for phenolic parts should be 6 mm (") wide and 0.08 mm - 0.09 mm (0.003" - 0.0035")

deep and vents for polyester parts should be 6 mm (") wide and 0.05 mm - 0.06 mm (0.002" -

0.0025") deep. The width is not as critical as the depth. A vent that is 0.025 mm (0.001") or less, is

too shallow and may seal when the mold is closed. A vent that is 0.13 mm (0.005") is normally too

deep and may not seal. As a result, internal cavity pressure will be low and the shrinkage, the

physical and the electrical properties may not match data sheet values.

Of equal importance to the location and depth of the vents is the vent length, which is the distance

from the part that the vent maintains its 0.08 mm (0.003") depth. The vent should be approximately

25 mm (1") long to allow pressure to build in the cavity after the material in the vent cures. After

this point, the vent can be relieved to a depth of 0.25 mm - 0.50 mm (0.01" - 0.02"). To help the

vent stay with the part, the corner of the vent at the part edge can be radiused or chamfered.

It is sometimes necessary to vent dead areas of the mold with vented ejector pins. Before

adding the vents, an ejector pin should fit the hole in which it will operate within 0.025 mm

(0.001"). A flat is then ground on the diameter no deeper than 0.13 mm (0.005") for a distance that

will take the vent 3 mm (") below the fit length of the pin. Normally, the fit length should be 13mm - 16 mm (" - ") long. (See sketch below) In addition, the stroke of the ejectors should be

long enough for the entire vent plus 3 mm (") to come up above the bottom of the cavity. This is

so the vent can be self -cleaning or so an operator can blow the flash off the pins.


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Something that is often overlooked in venting is the polish. It is recommended that all vents be

draw polished in the direction of flow to at least the same finish as the cavities and cores. They

should be polished for their entire length including the relieved distance. If a mold is to be chrome

plated, all the molding surfaces should be polished and plated including the vents.

Vacuum Venting

Some part designs are difficult to vent because of dead pockets or for other reasons. Also,

some materials, such as thermoset polyesters, are difficult to adequately vent using conventionalventing methods. In these situations vacuum venting is a good option to consider.


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In a vacuum vented mold, the cavities are sealed inside of a vacuum chamber with an O-Ring. A

vacuum of at least 21 in. of Hg is then drawn on the cavities. NOTE: A Venturi type vacuum

pump will NOT be able to obtain this level of vacuum in the cavities.

To check the amount of vacuum present in the mold cavities, we suggest closing the mold,

inserting a plug in the transfer pot, putting a vacuum gage into the plug, activating the vacuumand then timing how long it takes to reach the maximum vacuum reading. This timer

information is used to set the transfer delay so that once the vacuum is drawn, the molding

compound can be transferred into the cavities. NOTE: Having an accumulator tank in the

vacuum system will significantly decrease the amount of time needed to evacuate the cavities.

As can be seen in the sketch, the vacuum ports are located as far from the vents as possible. This

is to prevent material from being drawn through the vents and plugging a vacuum port. The

second vacuum port is a back up, in case the original port becomes blocked or plugged. NOTE:

The vacuum system needs an inline filter between the mold and the vacuum pump, to trap any

volatiles which would plug or damage the pump.

An O-Ring material that we have used successfully is high temperature silicone rubber that has a

60 to 70 durometer. One source of this material is McMaster Carr. Another source is Apex

Molded Products Company, 3574 Ruth St., Philadelphia, PA 19134-2094 and their telephone

number is (215) 289-4400 or (800) 221-8921.

A sketch for an O ring groove is shown below and is designed to hold the O ring in place and

not have it pulling out of the mold with each shot.


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NOTE: The diameter shown in the sketch below is 0.270". However, other diameters can be

used, as long as the proportions of the channel dimensions to the O-Ring dimension are

maintained.

If you have any questions about the design of the groove or how vacuum venting can be

incorporated into an existing mold, please contact the Technical Service Group of Plastics

Engineering Company.

ADDITIONAL MOLD DESIGN TIPS

Runner Design - When designing runners for molds, there are a number of possible approaches.

These include the standard full round with a centerline.

This is the most efficient runner, but in some cases it is

necessary for the runner to be in only one half of the

mold. See the figure to the left.


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A standard trapezoid runner is often used in situations

that require the runner to be only in one mold half. The

effective runner size is shown in the figure to the left. The

four corners become "dead" areas with very little material

movement. See the figure to the left.

To reduce the amount of scrap in the runner, a modified

trapezoid runner design is suggested. This design reduces

the dead areas without a significant change to the

effectiveness of the runner. See the figure to the left.

Gates - The gates for thermoset molds are high wear areas of the mold and therefore, need to be

designed with this thought in mind. The gate should be made using a replaceable insert so when

the gate becomes badly worn it can easily be replaced. A gate should be made of materials that

do not wear easily. Three materials commonly used for gate inserts are carbide, D-2 steel and

CPM-10V particle steel made by Crucible Steel.

In addition to inserting the gate, we find that inserting the mold opposite the gate and at the

impingement area in the cavity are beneficial. These areas are also high wear locations and will

need some maintenance as the mold is run.

When designing an edge gate for thermoset materials, the width of the gate can be as small as

1.5 mm (1/16") but the depth of the gate should not be less than 1.3 mm (0.050"). A gate should

be large enough to allow the part to fill without using excessive transfer pressures or requiring

long transfer times. Transfer times of 3 - 8 seconds and transfer pressures of 7.6 MPa (1,100 psi)

or less are desirable. Avoid using multiple gates on parts to minimize the number of knitlines. A

knitline is created went two fronts of material meet. Knitlines are weaker than the rest of the

part because there is not as much crosslinking that takes place across the knit as there is in the

main body of the part. To keep the overall strength of the parts as high as possible, the number

of knitlines should be kept to a minimum.


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A second type of gate that is

widely used in molds processing

thermoset materials is the

subgate. This type of gate is

sometimes referred to as a tunnel

gate. The advantage of a subgate

is it shears off as the part is

ejected from the mold. As a

result, there is no need for a

secondary operation to remove

the gate nor is there any concern

that the gate will project out from

the part and become an assembly

or a visual problem. In addition to the gate removal feature, the subgate can sometimes be

designed to direct the flow of material towards a difficult to fill location. In this way, the part

can be made easier to fill, which can have a positive effect on cycle times and scrap rates. Gate

size is dependent on the size of the part. Typically 0.13mm (0.050") can be used for small partsand 0.20mm (0.080") for larger parts. There are some problems associated with using subgates

that include:

The tip of the gate breaking off and sticking in the mold. This is especially true for polyester

molding materials and therefore, the use of subgates in molds for polyester parts is not

recommended.

The amount of steel at the parting line above the gate being too thin which results in the

metal wearing away very quickly after the mold begins to produce parts.

To reduce the likelihood of the gate tip breaking off and sticking in the mold, the tunnel needs tobe well polished so all EDM pits are removed. Locating an ejector pin at least 38 mm (1")

from the tunnel allows the runner to flex and pull the gate out of the mold without breaking. It is

also important to design the tunnel so the angle of incidence with the part allows the gate to pull,

but keeps sufficient thickness of steel at the parting line to prevent damage. See the sketch for

further clarification of how to design a subgate.

Recent developments in thermoset molding have shown that a part can be transfer molded with

nearly all signs of the gate gone. This is done using a gate cutter. A gate cutter is a blade or a

pin that is located in the mold directly below the gate. Immediately after the material is injected

into the cavity, this blade is advanced forward to seal off the gate. Once the blade is in the

forward position, the material cures against it, producing the same finish as the rest of the part.The only visible trace of a gate is a witness line.

Cavities and Core - In nearly all molds, the use ofinserted cavities and cores is encouraged.

The primary reason for this is in the event of an individual cavity or core being damage, that

particular cavity can be removed from the mold and repaired while the rest of the mold is put

back into service. Having individual cavities also allows for insert changes that make it possible


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to run multiple versions of the same basic part simultaneously. When the parts are very small

and there is a large number of cavities, individual cavity inserts might not be feasible. In those

situations, we would suggest using cavity inserts of 3 or 4 cavities. The materials most

commonly used for cavity inserts are H-13 and S-7. Both of these materials will harden to

Rockwell 52 to 54 Rc and can be polished to produce an excellent surface finish on the parts.

Ejector Pin Location and Design - Without ejector pins it is usually not possible to remove the

molded part from the mold. The placement of the ejector pins is almost as critical as the location

of the gate. The pins should push the part out of the mold without distorting it and without

leaving an objectionable mark on the part. A secondary reason for having ejector pins is to aid in

the venting of the mold.

Ejector pins should be located in the deepest points of the cavity or core. We specifically suggest

that ejector pins be located on the deepest points of ribs and bosses. If ejector pins are not

located correctly, the part has to be pulled out of the deeper areas or the mold. Parts that have

to be pulled out of the mold are more likely to stick or be distorted during ejection. (See sketch

below.)

Once the location of the ejector pins is determined, the pin size needs to be decided upon. Very

small diameter ejector pins can be problematic because of their susceptibility to breaking.

Therefore, ejector pins smaller than 2.4 mm (3/32") in diameter are not recommended. Another

common problem is material flowing down around the ejector pin and jamming it so it breaks

when the ejectors are actuated. To prevent this from happening, the hole for the pin should only

be 0.025 mm (0.001") larger than the pin for a depth of 13 mm - 16 mm (" - ") from the

cavity. Making it deeper can result in the pin binding and breaking.

To ensure that the ejector plate moves along the centerline of the ejector pins, it is suggested that

the mold be equipped with a guided ejector system. In addition to aligning the ejectors, theguided ejector system moves the load of the ejector plate and the retainer plate from the ejector

pins to the guide pins and bushings of the ejector system. While aligning the ejector holes in the

mold with those in the retainer plate is always important, with a guided ejector system the

alignment is even more critical.

While it is desirable to have the ejector pins located on flat surfaces, this is not always possible.

It is sometimes necessary to locate ejector pins on contoured surfaces. Ejector pins located on


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contoured surfaces should be made to match the contour of the cavity. It will be necessary to key

these pins so they will maintain their alignment with the contour of the cavity.

Polishing and Plating - The trend has been to cut back on polishing because of its high cost.

Molds are being made that still show cutter marks on the non-visible areas of the parts. While

this practice may save money in the construction of the mold, it may increase part costs due tohigh scrap and down time. The non-polishedareas will generate frictional heat in the material

as it passes over these areas. This added heat can cause the material to cure prior to filling the

part. These unpolished areas can change the filling pattern of the material, which can result in

gas being trapped in locations that can not be vented. For these reasons, it is suggested that all

molding surfaces be polished to a minimum of SPI #2 rating. The mold surfaces to be polished

include the cavities and cores, the vents, the gates, the runners, the transfer pot and the entire

parting line. The reason for polishing the parting line is to insure that any flash that may occur on

it will come off of the mold with a minimal amount of effort. When polishing a mold, care

should be taken to be sure to always polish in the direction of draw. Vents need to be polished in

the direction of material flow and they should have the same degree of polish as the cavity and

core. Flat surfaces that have no influence on the part removal can be polished in any direction.When polishing deep ribs that were cut using the EDM process, it is important to be sure to

polish out all of the EDM pit marks. Otherwise, there may be a problem with the rib breaking off

of the part and sticking in the mold.

After the mold is completely polished, it is ready to be plated. Please keep in mind that any

defect in the steel surface will not be covered by the plating, but will be accentuated by it. While

there are a number of different types of plating available, to date, chrome plated molds provide

the best part release with the best part finish. Because some materials have fillers that are

incompatible with nickel, the use of nickel or electroless nickel to plate molding surfaces is

discouraged. In addition, nickel plating lack the wear resistance of chrome plating.

The surfaces to be plated should include the cores, the cavities, the core pins, the ends of the

ejector pins, the runner blocks, the vents, and the entire parting line. To protect the molding

surfaces and to insure good part release, it is necessary to plate all the surfaces that were

polished. After the mold is plated, it will be necessary to repolish the chrome because

unpolished chrome plating may cause sticking.

Center Supports - Often we find that molds built to run thermoset materials have little or no

support in the middle. This will result is heavy flash around the sprue and parts that vary in

thickness from the sprue side to the side opposite. To resolve this problem we suggest installing

substantial support pillars down the center of the mold between the parallels (50.8mm (2) dia. if

possible).

High Centering the Mold - Sometimes the center of a mold will have heavy flash even with

good center support. In these cases it may be necessary to do what we call Doming the Mold

or High Centering the Mold. This is accomplished by placing a 0.0508mm 0.0762mm


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(0.002 or 0.003) shim on the support pillars in the center of the mold, which will cause the

moving side of the mold to be slightly domed.

Side Locks - Any molds where maintaining the alignment of the mold halves is critical to

meeting the quality requirements require non tapered side locks. They should be located on all

four sides of the mold. The overall design of Progressive Components side locks is very good,since they have a longer engagement and are thicker.

Date Printed: January 29, 2009Date Revised: January 7, 2009

Supersedes Revision Dated: October 9, 2008

This information is suggested as a guide to those interested in processing Plenco Thermoset molding materials. The

information presented is for your evaluation and may or may not be compatible for all mold designs, runner systems,

press configurations, and material rheology. Please feel free to call Plenco with any questions about PLENCO

molding materials or processing and a Technical Service Representative will assist you.

sect 23 transfer mold design tips

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